Conventionally, a multicarrier CDMA (Code Division Multiple Access) system is designed in such a way that a transmission apparatus spreads transmission data in a frequency axis direction and sends code-multiplexed data. The data transmitted is received by a reception apparatus under the influence of frequency selective fading, and therefore orthogonality among spreading codes is lost at the reception apparatus and the reception performance deteriorates. To reduce the loss of orthogonality among spreading codes and improve the reception performance, a method of improving the reception performance by carrying out despreading using an algorithm such as MMSE (Minimum Mean Square Error) is widely known (e.g., see Document 1 “Performance of Forward Link Broadband OFCDM Packet Wireless Access using MMSE Combining Scheme based on SIR Estimation” Institute of Electronics, Technical Report of IEICE, RCS2001-166, October 2001, pp. 105-111).
Furthermore, if the same power is set for respective subcarriers at the reception apparatus, orthogonality among spreading codes is not lost at the reception apparatus, and therefore a method is conceived whereby the transmission apparatus adjusts transmit power of subcarriers beforehand and sends the subcarriers so that reception power becomes equal for all the subcarriers at the reception apparatus (e.g., see Document 2 “Performance of predistortion techniques for uplink MC-CDMA systems with TDD and FDD modes, International Conference The Fifth International Symposium on Wireless Personal Multimedia Communications (WPMC'02) October 2002, pp. 655-662”).
However, the reception method applying the MMSE algorithm disclosed in Document 1 above requires the reception apparatus to measure noise power, which makes the configuration of the reception apparatus more complicated. Furthermore, depending on the condition of a propagation environment, it is difficult to completely recover the loss of orthogonality among spreading codes, which results in a problem that it is not always possible to obtain optimum reception performance.
Furthermore, according to the method disclosed in Document 2 above, the reception apparatus is designed to perform EGC (equal gain) combining, which results in a problem that the reception performance deteriorates, compared to the case where MRC combining is performed.
FIG. 1 is a block diagram showing the configuration of a conventional transmission apparatus 10. In the transmission apparatus 10, transmission data directed to respective reception apparatuses are input to spreading sections 11 and serial/parallel (S/P) conversion sections 12, the number of which is equivalent to the code multiplexing number (number of reception apparatuses).
The spreading section 11 performs spreading processing on the transmission data using a predetermined spreading code and then supplies the spread signal to the S/P conversion section 12. The S/P conversion section 12 converts the spread serial signal to parallel signals and creates, for example, four subcarriers and supplies the subcarriers to their respective adders 13-1 to 13-4.
The adder 13-1 adds up a first subcarrier which is output from a first combination of the spreading sections 11 and S/P conversion sections 12, the number of which is equivalent to the code multiplexing number (number of reception apparatuses) and a first subcarrier which is output from a second combination of the spreading sections 11 and S/P conversion sections 12. In the first subcarrier, a signal spread by a first spreading code for a first user (first reception apparatus) and a signal spread by a second spreading code for a second user (second reception apparatus) are added up and the first subcarrier is constructed in this way. This first subcarrier is supplied to a multiplier 14-1.
Likewise, the other adders 13-2 to 13-4 also perform additions between the second subcarriers, between the third subcarriers and between the fourth subcarriers output from the respective combinations of the spreading sections 11 and S/P conversion sections 12 corresponding to the respective users (reception apparatuses) and supply the results to multipliers 14-2 to 14-4. The multipliers 14-1 to 14-4 multiply the respective subcarriers by weighting factors calculated by a weighting factor calculation section 23 for the respective subcarriers.
The outputs of the multipliers 14-1 to 14-4 are supplied to an IFFT (Inverse Fast Fourier Transform) processing section 15. By superimposing the respective subcarriers, the IFFT processing section 15 generates an OFDM signal (multicarrier signal) and supplies this OFDM signal to a GI (Guard Interval) addition section 16. After adding a guard interval to the OFDM signal, the GI addition section 16 supplies the OFDM signal to a transmission RF (Radio Frequency) section 17. The transmission RF section 17 carries out predetermined radio transmission processing (e.g., D/A conversion and up-conversion, etc.) on the signal with the guard interval inserted and transmits the signal after this radio transmission processing through an antenna 18 as a radio signal.
The signal transmitted from the transmission apparatus 10 is received by a reception apparatus. FIG. 2 is a block diagram showing the configuration of a reception apparatus 30. The received signal received at a reception RF section 32 through an antenna 31 in the reception apparatus 30 is subjected to predetermined radio reception processing (e.g., down-conversion and A/D conversion, etc.) here. The reception RF section 32 supplies the signal after this radio reception processing to a GI elimination section 33.
The GI elimination section 33 removes the guard interval inserted in the signal after the radio reception processing and supplies the signal after the guard interval elimination to an FFT (Fast Fourier Transform) processing section 34. The FFT processing section 34 performs a serial/parallel (S/P) conversion on the signal after the guard interval elimination, carries out FFT processing on the signal after the S/P conversion, converts the signal to information pieces for the respective subcarriers and supplies pilot symbols which are known signals of the signal after the FFT processing to a channel estimation section 35 for the respective subcarriers.
The channel estimation section 35 performs channel estimation on the respective subcarriers using the pilot symbols for the respective subcarriers and supplies the channel estimation values obtained for the respective subcarriers to an EGC coefficient calculation section 36 and a control channel transmission section 39.
The EGC (Equal Gain Combining) coefficient calculation section 36 calculates EGC coefficients for carrying out equal gain combining for the channel estimation values for the respective subcarriers and supplies these EGC coefficients to multipliers 37-1 to 37-4. The multipliers 37-1 to 37-4 multiply the respective subcarriers after the FFT processing output from the FFT processing section 34 by the coefficients supplied from the EGC coefficient calculation section 36, supply the multiplication results to a despreading section 38, and thereby carry out ECG despreading processing.
Furthermore, the control channel transmission section 39 is intended to transmit the channel estimation values of the respective subcarriers supplied from the channel estimation section 35 through a control channel and supplies the respective channel estimation values to a transmission RF section 40. The transmission RF section 40 carries out predetermined radio transmission processing (e.g., D/A conversion and up-conversion, etc.) on the respective channel estimation value information pieces and transmits the signal after this radio transmission processing through an antenna 41 as a radio signal.
The transmission apparatus 10 (FIG. 1) which has received the signal transmitted from this reception apparatus 30 carries out predetermined radio reception processing (e.g., down-conversion and A/D conversion, etc.) on the received signal at a reception RF section 20 and supplies the signal after this radio reception processing to a control channel reception section 21. The control channel reception section 21 extracts a control channel from the received signal and supplies data of this extracted control channel to a channel information detection section 22.
The channel information detection section 22 detects the channel estimation values of the respective subcarriers transmitted from the reception apparatus through the control channel as feedback information and supplies these channel estimation values to the weighting factor calculation section 23. The weighting factor calculation section 23 calculates weighting factors from the channel estimation values of the respective subcarriers and supplies the calculated weighting factors to the multipliers 14-1 to 14-4. As these weighting factors, the reciprocals of the channel estimation values of the respective subcarriers are used as shown in FIG. 3. In this way, the transmission apparatus 10 transmits subcarriers having small reception power at the reception apparatus 30 with increased transmit power, while the transmission apparatus 10 transmits subcarriers having large reception power at the reception apparatus 30 with decreased transmit power, thus allowing the reception apparatus 30 to receive the respective subcarriers with constant reception power. Assuming that reception power is fixed, the reception apparatus 30 despreads the signal by restoring only a change in the phase (equal gain combining type despreading), and can thereby recover orthogonality among spreading codes even if there is frequency selective fading.
As shown in FIG. 4, in the conventional transmission apparatus 10 in such a configuration, the respective subcarriers (#1 to #4) have different transmit power values due to weighting, but the transmit power is fixed for all spreading codes (all users) in each subcarrier.
As shown above, a conventional multicarrier CDMA system using EGC (equal gain combining) carries out weighting for each subcarrier and only equalizes power of received signals at a reception apparatus, and therefore it is difficult to optimize an SNR (Signal to Noise Ratio) though the reception apparatus can recover orthogonality among spreading codes.